1. Field of the Invention
The present invention is directed to a method for determining a weight with a dynamic scale according to an arrangement for the implementation of the method particularly for fast mixed mail processing in a franking system. The method is suitable for users of a mail processing system with a dynamic scale and a postage-calculating postage meter machine, or a system with a dynamic postage-calculating scale and a franking unit.
2. Description of the Prior Art
A digital scale is disclosed in German PS 37 35 036, wherein the analog output signal of a weighing cell is converted into a digital signal in an analog-to-digital converter. A microprocessor compares this digital signal to a zero value in order to derive the weight.
U.S. Pat. No. 4,956,782 and British Specification 22 35 656 disclose semi-dynamic weighing. A user station arranged upstream in a mail flow can be a scale and a user station arranged downstream can be a postage meter machine. A continuously moved piece of mail must be completely accepted by the scale for the measuring time required for the weight determination. Given mixed mail, the probability is greater that the scale has determined an incorrect measured result. Dependent on the dimensions of the letter, the measuring time is lengthened. Alternatively, the weighing length could in fact be increased and a reject compartment could be provided, however, this would increase the length of the overall mail processing system, which is not possible without significant refitting. If incorrectly measured letters are not to be rejected, the conveying speed of the letters on the scale must be reduced. A complicated control and controllable motors are required for this purpose.
According to European Application 514 107, transport is interrupted by a control means given large pieces of mail that are difficult to weigh, until the measurement is stable. A detector is arranged in the scale close to the conveyor belt at a downstream location, this detector only allowing letters to pass for which the weight has been identified before reaching the detector. Given unequally distributed mass in the letter and high conveying speed, measuring errors occur. Moreover, the letter can slide off the weighing pan due to its inertia given a stoppage. The dimensions of the weighing pan are therefore designed somewhat greater or the conveying speed is fixed lower that would otherwise be necessary. The throughput given mixed mail is correspondingly reduced.
German Published Application 37 31 494 (U.S. Pat. No. 4,753,432) discloses that the weighing procedure include an idle time for the actual weight measurement, whereby the operation of the franking system and the transport system are interrupted because continued operation would otherwise supply too large a vibration. The conveying time from the weighing location to the postage meter machine is selected short, however, the speed cannot be arbitrarily increased without increasing the risk of jamming. The item output that can be achieved is limited by pauses introduced into the executive sequence. The speed of the weighing is limited by the speed of the weight determination. The latter factor presents a greater outlay given heavy pieces of mail when the measurement must be exact.
German Published Application 7 31 508 (U.S. Pat. No. 4,787,048) discloses that a weight estimation be undertaken when weighing. Proceeding from weight-graduated postage fee schedule tables, a first approximation is initially undertaken to assign the weight item to one of the weight categories and the separation thereof from a next category is identified. If the separation is adequately large, then this first approximation can be employed in order to determine the postage fee.
A method for dynamic weighing is known wherein interpretation of successive maximums takes place, with the weight value being formed from a difference derived from the first maximum and the quotient of the difference between the first maximum and the second maximum, and a value that is dependent on the system-inherent damping and on the period duration (Goldner, Hans, Leitfaden der Technischen Mechanik: Statistik, Festigkeitslehre; Kinematik, Dynamik, Chapter 6.2.2, Freie Schwingungen gedampfter Systeme, pages 599 through 606, VEB Fachbuchverlag Leipzig 1986, revised 9th edition). A disadvantage of this procedure is that one must wait at least half the period of a possibly-occurring harmonically decaying vibration. Since the curve form in practice, however, is not always ideal, the actual period duration can only be derived after several periods. A high degree of measuring uncertainty exists given measuring curves that do not proceed ideally.
None of the aforementioned solutions have any possibility for adaptation to a mail processing system that operates with a high throughput of postal matter. Due to the different conveying speeds, a modified behavior of the piece of mail during transport across the weighing device can occur. The piece of mail deforms more greatly at the edges during handling, air cushions are differently formed, impacts are later exerted on the weighing device, etc. Due to the mass moment of inertia, a piece of mail retains motion longer after transfer from a delivery means if the delivery does not ensue exactly. A different curve form of the measured curve for the measured weight values thus occurs as a result and, given a higher conveying speed, fewer measured weight values are available given the same structural length of the scale.